Micro-encapsulation denotes enclosing fine particles or liquid droplets with a film wall to obtain microcapsules having useful characteristics. Frequently unstable substances are micro-encapsulated, and the obtained microcapsules retaining the properties of the substance are used for various applications.
In the fields of recording or display materials, aromas, adhesives and the like, micro-encapsulation was once conducted chiefly by the coacervation method. However, a new micro-encapsulation method was recently proposed in which a film wall of amino resin is formed by in-situ polymerization. Now the new method is predominantly performed because the method uses inexpensive materials, comprises expediently simplified procedures and gives microcapsules of proper strength. Using a specific water-soluble polymer (system modifier), the method is capable of smoothly forming a film wall of the amino resin. Therefore, the method has been variously improved mainly in the selection of system modifiers. For example, improved methods have been proposed as disclosed, e.g., in Japanese Unexamined Patent Publications Nos. 25277/1979, 107881/1979 and 51238/1981.
Yet these conventional film walls of amino resin are hard and brittle and thus likely to develop cracking. Since the development of cracking is accelerated by heat, water or the like, the conventional microcapsules with a film wall of amino resin have the drawbacks of being low in resistance to heat and moisture and tending to ooze out the contents during storage.
Japanese Unexamined Patent Publication No. 170857/1984 discloses a micro-encapsulated toner covered with two walls, namely a primary wall of a resin prepared by addition reaction and/or condensation reaction of a component with a formaldehyde and a secondary wall formed on the primary wall by coacervation of polymer from an organic solution. The micro-encapsulation disclosed in this publication entails complicated steps because of required batchwise formation of the primary and secondary walls and necessitates the formation of the secondary wall in the organic solvent in which the contents are apt to dissolve out during the formation thereof or to have the properties impaired. Further the obtained microcapsules are unsatisfactory in resistance to heat and moisture.